1. Field of the Invention
The present invention relates to energy absorbing bumpers, and more specifically to energy dissipating structures embodied in such vehicle bumpers.
2. Description of Related Art
Energy absorbing bumpers have been used on automotive vehicles for many years to attenuate collision energies. Flexible polymeric materials and plastic foams have been employed as energy isolation members to dissipate these energies. Generally, energy isolation members employing foam have been mounted to the front and rear of a vehicle and covered externally by a flexible cover. In the event of a collision, the object contacting the vehicle elastically stresses the flexible cover laterally in tension and deflects it inward to the vehicle where it contacts the energy isolation member. The foam energy isolation member is subsequently compressed wherein the kinetic energy of the vehicle is given up to mechanical work and heat through thousands of mechanical contacts within the foam; the energy is dissipated before it can be transferred to the vehicle body structure. After the vehicle comes to rest, contact ceases between the vehicle and the outside object, and both the energy isolation member and the flexible cover rebound to substantially their original shape.
U.S. Pat. No. 4,268,078 to Nomura is exemplary of bumpers like the one described above. Typically, these bumpers perform better during collisions with objects having relatively large projected contact area, like walls and other vehicles. Larger volumes of foam are involved in dissipating collision energies when presented by the large contact area of a wall; more mechanical contacts within the compressed foam results in lessened opportunity for kinetic energy to be transferred to the vehicle body structure. However, this type of bumper performs less optimally when colliding with objects having smaller projected contact area, like poles or the edges of buildings. The energy isolation member is compressed locally about the area of load application leaving much of the foam uninvolved in the collision. Less foam is available to convert the vehicle's kinetic energy to mechanical work and heat. Consequently, the kinetic energy is converted to mechanical work within the body structure of the vehicle which may result in plastic deformation of sheet metal body components.
The condition cited above is effected because smaller volumes of foam are involved in changing the vehicle's momentum. Collision contact time with the energy isolation member is greatly reduced. Impulsive forces, directed substantially longitudinally inward to the vehicle, are directly transmitted as sudden impacts. Coupling the impulsive loads with the reduced contact area increases compressive stresses on the foam material, and may in some cases lead to a complete collapse, and possible shearing, of the foam energy isolation member locally about the area of contact. Consequently, and regardless of shearing, the impulsive loads perform mechanical work on the sheet metal body components leading to plastic deformation in a typical "wraparound" pattern.
It would be desirable to develop a device for a vehicle bumper that would dissipate the energy of collisions with poles and the like through a larger volume of energy absorbing material than would otherwise be possible.